There are many dual fed systems with local generation. Typically, these systems comprise a first busbar, a second busbar, at least one generator connected to the first busbar and having an associated prime mover, at least one generator connected to the second busbar and having an associated prime mover, and an electrical load connected to the first busbar by means of a first power converter and connected to the second busbar by means of a second power converter. Such dual fed systems may operate with alternating current or direct current depending upon the specific system.
Possible applications of dual fed system include industrial works that have local power generation, for example steel works. The electrical loads of these systems may comprise any or all mechanical operations of the works including, but not limited to, water pumps, fans, gas compressors, electric arc furnaces, rolling mill drives and crane drives. Another possible application of a dual fed system is a mine with local power generation wherein the electrical loads may be hoists, conveyers, crushers, fans or any other equipment of the mine.
One example of a dual fed system with local generation is the propulsion system of some marine vessels. These systems operate with alternating current. The electrical loads of the systems are the thrusters of the vessel. Generally, such propulsion systems are operated by dynamic positioning systems.
Dynamic positioning (DP) of marine vessels relies on the use of thrusters to maintain the position of a vessel in the vicinity of a reference point and stabilise its heading, in opposition to environmental forces such as wind and current. The object of a DP system is not to hold the vessel absolutely stationary, but to maintain its station within acceptable limits. The term “dynamic positioning (DP) system” is intended to include other positioning systems for vessels such as position mooring systems and thruster-assisted mooring systems which combine aspects of a DP system with a mooring system.
In a typical DP system the thrust demands from a controller are allocated to the available thrusters according to some definition of optimality. For example, thrust demands may be allocated to achieve minimum total power usage by sharing thrust in proportion to the rated power of each thruster. In a marine propulsion system the thrusters will be connected to ac busbars that receive power from ac generators that are coupled to prime movers, typically diesel engines or gas turbines that produce environmentally harmful exhaust gases such as nitrogen oxides (NOx), carbon dioxide (CO2) and other pollutants related to the combustion process.
Selective catalytic reduction (SCR) is a means of converting nitrogen oxides with the aid of a catalyst into nitrogen (N2) and water (H2O). A gaseous reductant, typically anhydrous ammonia, aqueous ammonia or urea, is added to a stream of flue or exhaust gases and is absorbed onto a catalyst. Carbon dioxide is a reaction product when urea is used as the reductant. Under the right preconditions, SCR is capable of eliminating over 85% of the nitrogen oxides produced during diesel engine combustion. In the hot exhaust gases the urea solution decomposes to ammonia and carbon dioxide in the following reaction:CO(NH2)2+H2O→2NH3+CO2 
The ammonia converts the nitrogen oxides in the exhaust gases to nitrogen and water vapour in the following reaction:NOx,NH3,O2→N2,H2O
An unwanted secondary reaction produces sulphur trioxide (SO3) and ammonium sulphates from sulphur in the fuel. It is important to maintain the temperature of the exhaust gases at a level which prevents the deposits of sulphates which can “mask” the catalyst and adversely affect the NOx conversion efficiency of the SCR system. It is therefore common for diesel engines to include a bypass feature which means that when the generator that is coupled to the diesel engine is operating at low power compared to rated power a significant proportion of the exhaust gases bypass the SCR system and the NOx emissions per kilowatt-hour are increased.
Production of carbon dioxide by diesel engines or gas turbines is dependent on engine efficiency; where efficiency can be considered to be the ratio of the electrical energy output from an associated electrical generator and the calorific value of the fuel that is consumed by the diesel engine or gas turbine. At low efficiencies more carbon dioxide will be produced per kilowatt-hour of energy output. It is therefore advantageous to operate diesel engines and gas turbines at high efficiency to minimise carbon dioxide emissions.
Sharing thrust in proportion to the rated power of each thruster does not necessarily achieve minimum NOx emission during operation of the DP system (“Reducing NOx emissions in DP2 and DP3 operations”, B. Realfson, DP Conference, 13-14 Oct. 2009, Houston, Tex.). One way of reducing NOx emissions is to share the thrust between isolated busbar sections of the marine propulsion system to load up some generators (and hence load up the coupled diesel engine) and unload others according to the predicted emission rates. Although this reduces NOx emissions it increases the total power usage. FIG. 1 shows a prior art marine propulsion system where thrusters T1-T4 are connected to ac busbars 30a, 30b that are interconnected by a tie 32. A pair of ac generators G1, G2 are driven by associated diesel engines D1, D2 and supply ac power to the first ac busbar 30a. A pair of ac generators G3, G4 are driven by associated diesel engines D3, D4 and supply ac power to the second ac busbar 30b. Each thruster T1-T4 is connected to a single ac busbar by means of an associated power converter 34 that has a diode front end (i.e. a pair of passive rectifiers) and transformers. In order to alter the power taken from any of the generators G1-G4 along the lines suggested above then the ac busbars 30a, 30b must be isolated by opening the tie 32 and the thrust allocated to each thruster T1-T4 must be altered. As mentioned above, the minimum power solution for thrust allocation requires that thrust is shared in proportion to the rated power of each thruster and this is not possible in the marine propulsion system of FIG. 1.
FIG. 2 shows an alternative prior art marine propulsion system that is similar to the marine propulsion system of FIG. 1 but each thruster T1-T4 is now connected to two ac busbars. More particularly, a first passive rectifier 36 associated with each thruster is connected to the first ac busbar 30a and a second passive rectifier 38 is connected to the second ac busbar 30b. The ac busbars 30a, 30b are interconnected by a tie 32 which includes an inter-bus transformer 40 that provides a phase shift. The phase shift between the ac busbars allows the passive rectifiers 36, 38 to operate in 12-pulse mode, i.e. there are 12 voltage pulses from commutation of the diodes for each cycle of the ac supply voltage. However, the diode front end of each power converter 34 still does not allow control of the amount of power that is supplied by each ac busbar. Power sharing or allocation is dependent on the natural commutation of the passive rectifiers 36, 38 and will almost always share power equally from the first and second ac busbars 30a, 30b. 
An improved dual fed marine propulsion system that employs active front end (AFE) power converters is shown in FIG. 3. The marine propulsion includes a first ac busbar 2a and a second ac busbar 2b. The first and second ac busbars 2a, 2b may carry a low voltage (LV) supply voltage (e.g. 690 V) and may optionally be divided into separate individual sections.
A pair of ac generators G1, G2 are driven by associated diesel engines D1, D2 and supply ac power to the first ac busbar 2a. A pair of ac generators G3, G4 are also driven by associated diesel engines D3, D4 and supply ac power to the second ac busbar 2b. The generators G1-G4 are connected to the respective ac busbar by protective switchgear 4 with circuit breakers and associated controls or other switching means. It will be readily appreciated that the marine propulsion system may have any suitable number of ac generators and any suitable busbar configuration depending on the power generation and distribution requirements.
The ac busbars 2a, 2b may be interconnected by a tie 6.
The marine propulsion system includes a series of four parallel propulsion drive systems 22a-22d. Each propulsion drive system includes a thruster T1-T4 connected in parallel to the first and second ac busbars 2a, 2b by AFE power converters 8, 10 and associated harmonic filter systems 12. More particularly, a first AFE power converter 8 is connected between each thruster and the first ac busbar 2a and a second AFE power converter 10 is connected between each thruster and the second ac busbar 2b as shown in FIG. 3. Each AFE power converter includes a first active rectifier/inverter 14 (or ‘front end’ bridge) having ac input terminals connected to the respective ac busbar 2a, 2b and a second active rectifier/inverter 16 having ac output terminals connected to the thruster. The thruster is therefore connected to the ac output terminals of the second active rectifier/inverter 16 of each associated AFE power converter 8, 10 in parallel. The dc terminals of the first and second active rectifier/inverters 14, 16 for the first AFE power converter 8 are connected together by a dc link 18 and the dc terminals of the first and second active rectifier/inverters 14, 16 for the second AFE power converter 10 are connected together by a different dc link 18.
The ac input terminals of each first active rectifier/inverter 14 are connected to the associated ac busbar 2a, 2b by protective switchgear 20.
Although only shown for the first propulsion drive system 22a, the ac output terminals of each second active rectifier/inverter 16 are connected to the associated thruster T1-T4 by fast-acting isolation contactors 24 that are an optional feature. The first and second AFE power converters 8, 10 are also short circuit proof with the ability to shutdown safely and automatically in the event of a short circuit at their ac terminals.
In normal operation, the first active rectifier/inverter 14 will operate as an active rectifier to supply power to the dc link 18 and the second active rectifier/inverter 16 will operate as an inverter to supply power to the thruster, but reverse operation may be possible in certain circumstances such as regenerative braking where power is supplied from the thruster (operating as a generator) back to the ac busbars 2a, 2b. 
Each active rectifier/inverter 14, 16 may typically have a conventional three-phase two-level topology with a series of semiconductor power switching devices (e.g. IGBTs) fully controlled and regulated using a pulse width modulation strategy. However, in practice the active rectifier/inverters can have any suitable topology such as a three-level neutral point clamped topology or a multi-level topology, for example.
Additional ac busbars may be connected to ac busbars 2a, 2b by transformers so that the distribution voltages carried by the additional ac busbars are conveniently derived by transformer action. The additional ac busbars may be used to provide power to other electrical loads.
The thrusters T1-T4 may be of any suitable type and construction and are configured to drive a propeller shaft (not shown).
FIG. 4 shows an alternative marine propulsion system. The basic overall arrangement is similar to the marine propulsion system of FIG. 3 and like parts have been given the same reference numerals. The marine propulsion system includes two main thrusters (or larger propulsion motors) T1, T2 typically rated at 3.5 MW each and two smaller maneuvering thrusters T3, T4 which are particularly suitable for DP and are typically rated at 1.2 MW each. Each thruster T1-T4 is connected to first and second busbars 2a, 2b by AFE power converters as described above. During transit of the marine vessel the maneuvering thrusters T3, T4 are not required and the main thrusters T1, T2 receive power from the first ac busbar 2a by means of the AFE power converters 8a, 8b and 8c, 8d and from the second ac busbar 2b by means of the AFE power converters 10a, 10b and 10c, 10d. In other words, each of the main thrusters receives power from four AFE power converters, two being connected to the first ac busbar 2a and two being connected to the second ac busbar 2b. For DP operation the main thrusters T1, T2 will not require full power and can therefore receive power from any two of the four associated AFE power converters. For example, the first main thruster T1 can receive power from the first and second AFE power converters 8a, 10a leaving the third and fourth AFE power converters 8b, 10b to supply power to the first maneuvering thruster T3 or vice versa. The thrusters T1-T4 are connected to the associated AFE power converters by suitable switching means 26 that can select whether power from the second active rectifier/inverter 16 of each AFE power converter 8, 10 is connected to the main thruster or the maneuvering thruster of each propulsion drive system.
The marine propulsion system of FIG. 4 significantly reduces the cost of power electronics and reduces switchboard size, weight and cost Single point failure conditions are much reduced compared to conventional arrangements with increased fault tolerance since a fault in any of the AFE power converters will not affect both of the ac busbars 2a, 2b. The arrangement makes best use of installed power electronics and is particularly suitable for marine vessels where size and weight are important design considerations.
It will be readily appreciated that in the dual fed AFE arrangements shown in FIGS. 3 and 4 each thruster T1-T4 can be supplied with power from both ac busbars 2a, 2b by means of the associated AFE power converters 8, 10 at any given ratio. The ratio may alter during normal operation of the marine propulsion system or during fault conditions, for example, to utilise the power that is available from the ac busbars 2a, 2b. This provides increased flexibility and redundancy and can be advantageously exploited by a DP system to minimise exhaust emissions from the diesel engines or other prime movers (not shown) as described in more detail below.